First multisite setup: Is this possible?

Hi all

I'm trying to set up my first multisite, but I'm uncertain how to solve some problems:

I want an external site (on foobar.com) with news, pages etc. It should only be editable by admins.
I want an internal site (on internal.foobar.com) with a forum, event calendar etc, and access control. Most of the content here will be created by normal users.

I also want the external site to fetch events accepted by its admins from the internal site, and the internal site to fetch news from the external page and use them as forum threads.

I think I want people who are logged in to the internal page to see extra stuff on the external site - most notably a "Discuss this in the forums" link on the bottom of news/etc, but they should not see admin controls on the external page.

I want completely separate themes and I think I want seperate modules.

To summarize:
News, Pages, etc. should be created from the external site by the external admins.
Forum posts, events, etc. should be created from the internal site by ~anyone, but events should be promotable to the external page by external page admins.

How would I go about to accomplish this? Should I share lots of dbs, and if so - which? Should it really be just one site with the domain module?

Hope you can help me out a bit,
Daniel